The replacement of doped polycrystalline silicon as a gate and interconnection material in very high speed integrated circuits (VHSIC) with other materials having a lower sheet resistivity and, therefore, increased speed, is of substantial interest in the electronics industry. A material which has been used in such devices is a bi-layer structure composed of a refractory metal silicide overlying a layer of doped polycrystalline silicon, commonly referred to as a "polycide" structure.
It is desirable that the gate and interconnection dimensions in VHSIC devices be as small as 0.5 micrometer. It is difficult to achieve the degree of profile control required to etch a structure to such fine dimensions when the structure is comprised of two or more layers of different material because one layer may etch anisotropically while the other etches partially isotropically due to differing etch mechanisms and rates. This can produce undercutting, particularly if substantial overetching is required to remove conductive residues called "stringers" which commonly form on vertical walls during etching. Achieving proper profile control becomes even more difficult when the substrate has epitaxial silicon islands or other topographical features having vertical walls. When a layer of polycide is etched to form a line overlying such a topographical feature, the potential for stringer formation is substantially increased.
Finally, it is necessary that there be an acceptable degree of etch selectivity between the silicon layer of a polycide structure and the underlying substrate layer. It is frequently the case that polycide structures overlie layers of gate oxide which are very thin, e.g. 200 angstroms. Unless there is sufficient etch selectivity, the overetching necessary to assure complete removal of stringers may etch through such thin layers of gate oxide causing any device made from the structure to short out due to contact between the polycide material and conductive layers underlying the gate oxide. An improved etching process for polycide materials is provided in accordance with this invention.